When drilling offshore for oil production in extremely deep water and in areas where extremely high currents are encountered, the oil industry faces significant challenges in using conventional riser/mud systems. Some of the conditions that lead to difficulties are very high hydrostatic pressure. For example, in a deep formation when a bit penetrates below the mudline, the drill formation is exposed to a full mud column pressure, whereas the pore pressure is only equal to a column of sea water. An additional difficulty is the expense and size of the rig and equipment required for use to drill wells in extremely deep water. The necessity of additional and larger diameter casing strings requires a larger riser. The result of the increased riser is more wasted mud volume and greater tension and loading on the equipment. The greater tension and loading results in a requirement for larger drilling rigs.
Proposals have been made to drill without a riser. A subsea drilling head rotates with the drill pipe and seals the annulus. Drilling mud returns through a conduit to the surface. These proposals have not yet been put into practice.